feat(stdlib): Crypto.* — SHA-256 + HMAC-SHA256, constant-time verify (#19)

The security-sensitive counterpart to the fast, non-cryptographic Hash.*
library: standard, test-vector-backed hashing for signed saves and message
integrity, kept in its own namespace so nobody reaches for the wrong tool.

  Crypto.sha256(s)                SHA-256 -> 64-char lowercase hex
  Crypto.hmac_sha256(key, msg)    HMAC-SHA256 -> 64-char hex
  Crypto.verify_hmac(key, msg, mac)  recompute + constant-time compare -> bool
  Crypto.hex(s)                   lowercase hex of a string's bytes
  Crypto.ct_equal(a, b)           constant-time string equality

The primitives are implemented from scratch in plain integer LLVM IR
(FIPS 180-4 / RFC 2104): no libc crypto, no data-dependent branches in the
compression rounds, so a given input hashes to the same 32 bytes on every
platform and run. Digests are returned as hex strings, not raw bytes, because
a `str` is null-terminated and a raw digest can contain a NUL. MAC checks use
a non-short-circuiting compare so timing does not leak how much of a forged tag
was correct.

Emitted on demand via g_uses_cryptort, mirroring the emit_hash prelude gate.
Scoped to the deterministic, known-answer-testable core; OS-backed
random_bytes (the one piece that can't be validated by test vectors) is left
for a follow-up.

Tested against published SHA-256 vectors (empty/"abc"/fox + 55/56/64-byte
multi-block padding) and HMAC-SHA256 vectors; wired into the self-host suite as
`crypto`. Docs: a new Crypto section with honest "what this protects / does
not" guidance, one page per method, all fences checked and in the inventory.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/emit_core.ludic

@@ -25,6 +25,7 @@ var g_uses_mathrt: bool = false  # Math.sqrt/sin/cos/tan was emitted -> emit the
 var g_uses_textrt: bool = false  # Text.upper/lower/trim/repeat/pad was emitted -> emit the text builders
 var g_uses_textrt2: bool = false  # Text.split/join/replace was emitted -> emit the string/slice builders
 var g_uses_hashrt: bool = false  # Hash.of/fnv1a/crc32 was emitted -> emit the byte-stream hashers
+var g_uses_cryptort: bool = false  # Crypto.* was emitted -> emit the SHA-256 / HMAC runtime
 var g_uses_datert: bool = false  # Date.*/DateTime.* was emitted -> emit the civil<->epoch conversions
 var g_uses_longstr: bool = false  # string(long) / interpolating a long was emitted -> emit fn_long_str